The swing of the load suspended on a hoisting rope is a notable problem when a crane is used to handle materials. During the traversing motion, changes in the traversing speed always generate load swing of an amplitude depending on the length of the hoisting rope and the rate of speed change, i.e. acceleration. The elimination of load swing has been the subject of a great deal of investigation, and automatic systems to solve the problem have been developed. Examples of these can be found in FI patent 44036 (B66c 13/06) and conference publication Electric Energy Conference 1987, Adelaide, pp. 135-140. A feature common to these systems is that the goal of the traversing movement is already known at the moment of starting. An optimal speed profile is computed for the movement, and if this speed profile is observed, no swing occurs at the end of the movement and the time consumed to perform it is minimized.
In crane drives in which the traversing movement is controlled by the operator, damping load swing by the methods presented in the above-mentioned references is only possible if the operator works in accordance with certain conditions:
the operator changes the traversing speed setting in a stepwise manner to the desired speed at the start of the motion, PA1 the operator maintains the same speed setting for at least a minimum time depending on the height of the load, PA1 the operator changes the speed setting in a stepwise manner when changing the target speed, and PA1 the operator performs no new control actions before the system has reached a condition with no load swing.
Previously known is a technique whereby the traversing movement of a crane is so controlled that the load is in a no-swing condition when a new speed setting is given. The traversing speed is changed by using two acceleration sequences of equal length and separated from each other by half an oscillation cycle.
The principle described above can also be improved in a way that enables it to work under an arbitrary speed setting. If the operator's control actions permit, i.e. if the conditions presented above are fulfilled, a "natural motion curve" minimizing load swing, defined in a manner described in the publications referred to above, is observed. However, if the operator performs arbitrary control actions, the crane has to obey them because the operator must have the best possible control over the machine. As a consequence of arbitrary control actions and in operational situations where the above conditions are not fulfilled, the "natural motion curve" cannot be observed. Therefore, the swing generated by the control of the traversing movement cannot be compensated.
When the crane is controlled by giving the trolley a speed setting, the quickest way of reaching the desired speed is to control the motor at maximal acceleration until the target speed is reached. However, according to the references, to achieve swing-free traversal, an acceleration sequence must be followed by a corresponding acceleration sequence half an oscillation cycle later, increasing the stopping time and distance. The acceleration of the trolley is proportional to the torque of its motor and further to the current. Because of the motor current limitations, a given acceleration limit cannot be exceeded. In addition, the control system and the operating environment often impose other limitations, such as a maximum speed limit.
When a load is being moved by a crane, the crane operator should always have a good feel for the system. Speed changes and swing damping have to take place quickly. The velocity of the load should not exceed the speed setting by a large margin, and the load and the parts of the crane, such as the bridge or the trolley, should not move in a direction opposite to the control. When the speed reference changes, the load speed has to change immediately in the direction required by the change in the reference, especially when the speed reference is diminished.
The distance required to stop the load should only be dependent on the speed of the load and it should not vary according to the situation which prevailed at the moment when the stopping request was given. The distance through which the load travels after the speed reference has been set to zero should be minimized.